In the
largest study of its kind, a Swedish group has determined that
actual autism rates probably have not changed in recent years,
even though diagnoses of autism cases continue to climb.

The
research, led by Sebastian Lundstrom and colleagues at the
University of Gothenburg, found that about 1 percent of those
in an ongoing study of twins met the criteria for having
autism, even though the number of officially diagnosed autism
cases in the country’s national health registry had climbed
steadily over a 10-year period. The power of the study,
published last month in the British Medical Journal, comes
from the fact that Sweden has comprehensive health records for
its population, and the research covered nearly 20,000 twins
whose families were asked about their symptoms, along with
diagnostic records for more than a million children born
between 1993 and 2002.

Because
the study counted autism diagnoses of children up to age 10,
it covered a period up until about 2012.

In a
recent telephone interview, Lundstrom said there is no reason
to believe the Swedish experience with autism is much
different from that in the U.S. or other nations, and he said
there is no evidence to suggest that twins have a different
rate of autism than the general population.

The
national registry in Sweden includes all the official
diagnoses for autism spectrum disorder, which more than
doubled from 0.23 percent in 1993 to 0.5 percent in 2002. That
rate is lower than the 1 percent prevalence found among the
twins, but that may be because the national registry uses a
conservative definition of the disorder. In another Swedish
study last year that looked at all diagnoses for autism among
teens living in Stockholm County, the autism diagnosis rate
was about 2.5 percent.

"My
personal view is that the autism rate might lie between 1
percent and 2 percent, depending on which lens you look at
autism through," Lundstrom said, "and our study
suggests the (actual autism rate) has been fairly steady for
decades."

The
official U.S. autism rate put out by the federal Centers for
Disease Control and Prevention is now 1 in 68 children, or
about 1.5 percent, based on an extrapolation from school
records and official doctors’ diagnoses in 11 communities.

The
official U.S. rate has more than doubled in recent years, from
0.67 percent in 2002 to the current rate, leading some to
believe that environmental toxins or other factors have
created an explosion of new cases. The official CDC statement
says cautiously, "We don’t know what is causing this
increase. Some of it may be due to the way children are
identified, diagnosed and served in their local communities,
but exactly how much is unknown."

Studies
like the one in Sweden argue that almost all the increase has
been due to greater awareness of autism and an expansion in
the number of children who get the diagnosis.

Nancy
Minshew, director of the University of Pittsburgh’s Center
for Excellence in Autism Research, says that she thinks the
actual autism rate may be higher than the one calculated in
the Swedish study — up to 3 percent — but she agrees with
the study’s main conclusion.

"My
view is that the autism numbers have gone up, but the number
of people who are affected is actually unchanged, which is
what they are saying in this paper."

She said
much of the growth in the official numbers has come in
children with higher IQs, including those with Asperger’s
syndrome. Current figures show that half of all children in
the U.S. diagnosed with autism have IQs above 85, the official
threshold for those who are not mentally retarded.

Minshew
said many of these children simply were not recognized as
having autism decades ago, and "people will often say to
me now, I know there were a couple kids in my class who must
have had this. Or people will say, ‘This is what my brother
must have had; we never understood why he failed his graduate
program.’"

Autism
is marked by three main characteristics: difficulty with
social relationships, problems with communication and
repetitive behaviors or narrowly focused interests.

Much of
the debate over the autism numbers has revolved around whether
something in the environment — air and water pollution,
pesticides or certain drugs, for instance — might have
triggered a rise in autism.

Brian
Lee, an epidemiologist at Drexel University in Philadelphia,
participated in the Swedish teenager study that found a
prevalence rate of 2.5 percent.

"A
lot of papers show that most of the (autism) increase is due
to diagnostic substitution and better awareness and those
sorts of factors, but I’m still not entirely convinced it’s
not a real increase." The increases could partly be
caused by environmental damage that causes new mutations in a
child’s genome, he said. "We happen to live in one of
the more polluted times in recent history."

But
Lundstrom, the Swedish researcher, isn’t buying that
argument. "I am convinced there are several environmental
factors that could affect autism," he said, "but I
don’t think they’re increasing. One issue that people have
raised is pesticides, like living next to a big cornfield, but
no one can convince me that pesticides were less toxic in the
1970s."

In
epidemiology, having a large population to study leads to more
powerful results, Minshew noted.

"I
think the Swedish research design was strong, and I think
their results were solid. When you have 20,000 twins and 1.1
million total people, you can get very strong results."